Gas absorption spectroscopy generally measures the presence and/or concentration of a species of interest in a gas sample by passing a light beam through the sample and detecting the absorption at wavelengths of a particular spectral absorption feature of the species of interest. Generally, such a feature is an absorption line that represents the frequency of light corresponding to vibrational, rotational or electronic transitions of molecules of the gas of interest. Tunable diode lasers provide many advantages for such gas absorption spectroscopy measurements in that the lasers can be tuned to the center of a spectral feature and can provide a relatively narrow signal to the width of the spectral feature.
Laser absorption spectroscopy can thus offer high speed and relatively high precision capabilities in order to detect a variety of trace gas species in gas samples at atmospheric pressures with relatively low cross sensitivity to other gas species or components. Tunable diode laser spectrometers are particularly suited to high sensitivity studies, in part, because they may be frequency modulated to reduce low frequency laser noise and electronic noise. In general, a laser spectrometer will include a frequency tunable laser that generates an illumination output beam that is directed through a sample cell that contains a gas mixture. The beam is then directed to an optical detector and the signal of the detector is demodulated to obtain an absorption-induced signal. This absorption-induced signal can be used to identify one or more species of interest within the gas sample.